of another experimental piece of apparatus is shown in Fig.
16. A pear-shaped bulb of German glass has near the small end an inner
concave negative pole, A, of pure silver, so mounted that its
inverted image is thrown upon the opposite end of the tube. In front
of this pole is a screen of mica, C, having a small hole in the
center, so that only a narrow pencil of rays from the silver pole can
pass through, forming a bright spot, D, at the far end of the bulb.
The exhaustion is about the same as in the previous tube, and the
current has been allowed to pass continuously for many hours so as to
drive off a certain portion of the silver electrode; and upon
examination it is found that the silver has all been deposited in the
immediate neighborhood of the pole; while the spot, D, at the far end
of the tube, that has been continuously glowing with phosphorescent
light, is practically free from silver.

[Illustration: FIG. 15.--PRESSURE = 0.00068 MM. = 0.9 M.]

The experiment is too lengthy for me to repeat it here, so I shall not
attempt it; but I have on the table the results for examination.

The identity of action of silver and aluminum in the first case, and
the non-projection of silver in this second instance, are in
themselves sufficient to condemn Dr. Puluj's hypotheses, since they
prove that phosphorescence is independent of the material of the
negative electrode. In front of me is a set of tubes that to my mind
puts the matter wholly beyond doubt. The tubes contain no inside
electrodes with the residual gaseous molecules; and with them I will
proceed to give some of the most striking radiant-matter experiments
without any inner metallic poles at all.

[Illustration: FIG. 16.--PRESSURE = 0.00068 MM. = 0.9 M.]

In all these tubes the electrodes, which are of silver, are on the
outside, the current acting through the body of the glass. The first
tube contains gas only slightly rarefied and at the stratification
stage. It is simply a closed glass cylinder, with a coat of silver
deposited outside at each end, and exhausted to a pressure of 2
millimeters. The outline of the tube is shown in Fig. 17. I pass a
current, and, as you see, the stratifications, though faint, are
perfectly formed.

[Illustration: FIG. 17.--PRESSURE = 2 MM.]

The next tube, seen in outline in Fig. 18, shows the dark space. Like
the first it is a closed cylinder of glass, with a central indentation
forming a kind of hanging pocket and almo